The first goal is an investigation of the rate of synthesis and degradation of arylsulfatase A (ASA) and arylsulfatase B (ASB) in cat brain. At appropriate intervals after the intracerebral injection of N-acetyl-(14C) glucosamine, (3H) leucine, and (14C) leucine, lysosome-enriched fractions are prepared and ASA and ASB isolated from soluble extracts by immunoprecipitation with monospecific antisera to these enzymes. The immunoprecipitates are radioassayed and the specific radioactivities of ASA and ASB determined. This study should indicate whether ASA and ASB are synthesized and turn over independently as two separate enzymes, or whether, as suspected, ASB may be derived from ASA during its degradation in vivo. This possibility will be further investigated in a study of the in vitro digestion of purified human brain ASA by an extract from rat kidney lysosomes. Certain physicochemical, catalytic and immunological properties of the partially degraded derivatives of ASA will be compared with those of human brain ASB to ascertain whether ASB could be derived by lysosomal digestion of ASA.